Machine reamer and reaming head for a machine reamer

ABSTRACT

The invention relates to a machine reamer, with a shaft-like base body ( 10 ) and a reaming head ( 12 ), projecting axially from the front face of the base body. According to the invention, the reaming head is in the form of a one-piece exchangeable cutting tip ( 12 ), which may be chucked into a tip seat ( 14 ) on the front face of the base body ( 10 ), in a coaxially self-centering manner, in order to maintain a high precision coaxialism, despite low material and construction requirements.

[0001] The invention relates to a machine reamer comprising a shaft-likebase body and a reaming head projecting axially from the end face beyondthe base body.

[0002] Machine reamers of this type are used for the precision workingof cylindrical bores with the goal of a high degree of accuracy indimension and form. The cutting work is done by the main cutting teethon the face of the conical chamfered portion, whereas the secondarycutting edges in the axially following guide portion serve to guide thetool. An important characteristic of the machine reamer is thecoaxialism of the reaming head with respect to the axis of rotation. Thecutting edges in so-called monoblock reamers are directly soldered intoa base body. The monoblock reamers including the base body must whenworn be completely exchanged. The use of multi-toothed monoblock reamerson machine tools is therefore extremely expensive. In order to avoidthis disadvantage and in order to obtain acceptable tool life adjustablereamers are today increasingly being used. Small adjusting paths,however, limit here also the achievable cost advantages.

[0003] Starting from this, the basic purpose of the invention is toreduce the material and construction expense in machine reamers and yetto achieve good machining results.

[0004] To attain this purpose the characteristics disclosed in Patentclaims 1 and 16 are suggested. Advantageous embodiments and furtherdevelopments of the invention result from the dependent claims.

[0005] The basic thought of the solution of the invention is that thereaming head is designed as a one-piece exchangeable cutting tip, whichcan be chucked without play and in a coaxially auto-centering mannerinto an end face of a radially expandable tip seat of the base body. Inorder to achieve this it is suggested according to a first modifiedembodiment of the invention that the exchangeable cutting tip has ashoulder, which projects axially beyond a rearward facing planarsurface, and which has at least three key surfaces, which are arrangedat preferably equal angular distances from one another, and supplementone another to form a regular frustum, and that the tip seat has atleast three inclined surfaces, which are arranged at preferably equalangular distances from one another, and are complementary to the keysurfaces of the sides of the regular frustum, and which terminate in aplanar end surface, against which axially abuts the rearward facingplanar surface of the exchangeable cutting tip in the assembled state. Aplay-free driving rotation between the base body and the exchangeablecutting tip is guaranteed with these measures.

[0006] A preferred embodiment of the invention provides that the tipseat has at least three clamping members, which are arranged atpreferably equal angular distances from one another, are separated byaxially and/or radially open spacing gaps, and each have one of theinclined surfaces, and the chucking surfaces of which clamping membersclamp a plane forming the planar end surface. The clamping members areadvantageously dimensioned in such a manner that they can be elasticallyexpanded relative to the axis of the base body like a cantilevered beamunder the action of the applied force. An improvement in this respect isachieved in such a manner that the base body has inside of the tip seatat least three axially and/or radially open expansion slots, which arearranged at preferably equal angular distances from one another, wherebythe expansion slots are advantageously arranged in the base area of therespective clamping members. In order to permit the clamping members toexpand, it is advantageous when the slope angle of the inclined surfaceson the side of the base body is smaller than the slope of the keysurfaces on the side of the tips. As an alternative, it is possible tochoose the slope angles of the inclined surfaces on the side of the basebody and of the key surfaces on the side of the tips to be the same. Inorder to enable an expansion in this case, the planar surfaces on theend of the base body and on the end of the tips must be spaced at adefined distance from one another in an initial joining position of theconnecting partners. The slope angle of the key surfaces and/or of theinclined surfaces is advantageously 5° to 10° relative to the axis ofthe base body.

[0007] A second advantageous modified embodiment of the inventionprovides that the auto-centering tip seat has a conical socket, which isarranged in the planar end surface of the base body, and has an internalpolygon-sided socket, which is arranged between the conical socket andplanar end surface, whereas the reaming head has an outer conicalsection, which projects coaxially rearwardly from a planar surface, andcan be placed into the conical socket, and has an outer polygon-sidedsocket, which is oriented between the outer conical section and theannular planar surface, and is complementary with respect to the innerpolygon-sided socket. The reaming head abuts also in this case in theassembled state with its annular planar surface against the planar endsurface of the base body. A preferred embodiment of the modification ofthe invention provides that at least two clamping members are provided,which are arranged at preferably equal angular distances from oneanother, and are separated from one another by spacing gaps, which arearranged in the area of the inner polygon-sided socket corners, and areaxially and/or radially open, the chucking surfaces of which clampingmembers together define a plane forming the planar end surface, and canbe elastically expanded relative to the axis of the base body under theaction of the applied force.

[0008] A third advantageous modified embodiment of the inventionprovides that the auto-centering tip seat has a fitting cylinder, whichis arranged in the planar end surface of the base body, and has an innerpolygon-sided socket, which is arranged between the fitting cylinder andthe planar end surface, whereas the reaming head has a cylindricalfitting pin, which coaxially projects rearwardly from an annular planarsurface, and can be placed into the fitting cylinder, and is overdimensioned relative to the fitting cylinder, and has an outerpolygon-sided section, which is oriented between the fitting pin and theannular planar surface, and is complementary to the inner polygon-sidedsocket. The reaming head axially abuts also in this case with itsannular planar surface in the chucked state against the planar endsurface of the base body. A feed cone is advantageously arranged in thetip seat between the fitting cylinder and the inner polygon-sidedsocket. For the same reason it is possible for the reaming head to havea feed cone, which is arranged near the end face of its fitting pin.

[0009] A fourth advantageous modified embodiment of the inventionprovides that the auto-centering tip seat has a conical socket, which isarranged in the planar end surface of the base body, whereas the reaminghead has a rearwardly coaxially projecting outer conical sectionprojecting from an annular planar surface and can be inserted into theconical socket. The tip seat has in this case furthermore at least twoclamping members, which are arranged preferably at equal angulardistances from one another, and are separated by axially and/or radiallyopen spacing gaps, and the end surfaces of which members together definea plane forming the planar end surface. It is thus possible that thereaming head rests with its annular planar surface in the chucked stateagainst the planar end surface of the base body, and that the clampingmembers are elastically expanded relative to the axis of the base bodyunder the action of the applied force.

[0010] The exchangeable cutting tip can be chucked advantageously intothe tip seat by means of a centrally disposed cap screw. As analternative, it is possible for the exchangeable cutting tip to bechucked in the tip seat by means of a tie bar, which centrally andaxially extends through the base body. The tie bar can thereby beprovided with a center bore for passage of a cooling lubricant.

[0011] A preferred embodiment of the invention provides that theexchangeable cutting tip consists of a hard, preferably sintered cuttingmaterial. Advantageous cutting materials are hereby HSS (a highperformance high-speed cutting steel), hard metal, CBN (cubicbornitrite), PKD (polycrystalline diamond) or ceramics. The exchangeablecutting tip carries, if necessary, a friction-reducing coating. In orderto be able to work expeditiously, the exchangeable cutting tips areadvantageously designed to be multi-toothed, whereby in order to improvethe work result an uneven tooth pitch is provided.

[0012] Relatively small exchangeable cutting tips, the guide portion ofwhich is advantageously chosen to be shorter than half of the tipdiameter, are sufficient with the measures of the invention.

[0013] A significant advantage of the invention is that in the case ofwear only the reaming head must be exchanged. The relatively expensivebase body remains. The measures of the invention assure that thecoaxialism of the exchangeable cutting tips is guaranteed. Thedetermining factor for this is the auto-centering inclined abutment andthe planar surface abutment of the exchangeable cutting tip in the tipseat.

[0014] The invention will be discussed in greater detail hereinafter inconnection with the exemplary embodiments schematically illustrated inthe drawings, in which:

[0015]FIG. 1a is a diagrammatic illustration of a machine reamer with anexchangeable cutting tip;

[0016]FIG. 1b is a diagrammatic view of the front face of the machinereamer according to FIG. 1a;

[0017]FIG. 2 is a diagrammatic illustration of the base body of themachine reamer;

[0018]FIG. 3a is a longitudinal cross-sectional view of the base bodyaccording to FIG. 2;

[0019]FIG. 3b is a view of the front face of the base body according toFIG. 2;

[0020]FIGS. 4a and b are a diagrammatic illustration and a top view ofthe reaming head designed as an exchangeable cutting tip;

[0021]FIGS. 5a and b are two cross-sectional views along thecross-sectional lines A-A and B-B of FIG. 4b;

[0022]FIGS. 6a to c are diagrammatic illustrations of a side view and alongitudinal cross-sectional view of the dowel pin of the machine reameraccording to FIGS. 1a and b;

[0023]FIGS. 7a to c are diagrammatic illustrations of a side view and alongitudinal cross-sectional view of the chucking nut of the machinereamer according to FIGS. 1a and b;

[0024]FIGS. 8a and 8 b are a detailed cross-sectional illustration and atop view of a machine reamer, which has been modified with respect toFIGS. 1 to 7;

[0025]FIGS. 9a and b illustrate an exemplary embodiment for producing anauto-centering force within the tip seat;

[0026]FIGS. 10a and b illustrate an exemplary embodiment, which has beenmodified with respect to FIGS. 9a and b, for producing an auto-centeringforce in the tip seat;

[0027]FIGS. 11a to c illustrate three further modified embodiments of abase body of the machine reamer in an illustration corresponding to FIG.2.

[0028] The machine reamers illustrated in the drawings have a base body10 designed as a shaft and a reaming head axially projecting beyond thebase body. The reaming head is designed as a one-piece exchangeablecutting tip 12, which can be chucked without play and coaxiallyauto-centering in response to an axially applied force into a front-facetip seat 14 of the base body 10. The exchangeable cutting tip 12, whichconsists of a sintered cutting material, in particular of hard metal,has several cutting teeth 16, which are arranged spaced from one anotherin circumferential direction, and the tooth pitch of which isnon-uniform. The cutting teeth 16 each have an inclined chamferedportion 18, which assumes during the machining operation the importantcutting work. The chamfer angle is 45° to 60° in the illustratedexemplary embodiments. The chamfered portion 18 transitions axiallyrearwardly into the guide portion 20, which has mainly a guidingfunction within the bores to be machined. As can in particular be seenin FIGS. 1a, b and 4 a, the length of the guide portion 20 is reduced toa minimum in the illustrated exemplary embodiments. The guide portion 20is defined at its rear end by a planar surface 22. The exchangeablecutting tip 12 has furthermore a shoulder 24, which projects axiallybeyond the planar surface 22, and which has three key surfaces 26, whichare arranged at equal angular intervals from one another and supplementone another to form an axial frustum. Thus, it is possible to anchor theexchangeable cutting tip without play in the tip seat 14.

[0029] The tip seat 14 has for this purpose three inclined surfaces 28,which are arranged at equal angular distances from one another and arecomplementary with respect to the key surfaces 26 of the tip-sidefrustum, and which terminate in a planar end face surface 30. The planarsurface 22 of the exchangeable cutting tip 12 engages against the planarsurface 30 of the tip seat 14 when forcibly inserted therein. Theinclined surfaces 28 are arranged on each clamping member 32 forming thetip seat 14, which clamping members 32 are circumferentially separatedby axially open spacing gaps 34 at equal angular distances from oneanother on the base body 10. The clamping faces of the clamping members32 together clamp a plane forming the planar end face surface 30. Theclamping members 32 are dimensioned in such a manner that they can beexpanded like elastic cantilevered beams relative to the base body axisunder the action of the axially applied force. An improvement in thisrespect is achieved by the base body 10 having expansion slots 36, whichare arranged at equal angular distances from one another inside of thetip seat in the root area of the respective clamping members 32, andwhich are open axially toward the end face (compare FIG. 8a). Theexchangeable cutting tip 12 has an axial bore 38 for facilitating afastening to the base body 10. The exchangeable cutting tip 12 is in theexemplary embodiment illustrated in FIGS. 8a and b chucked to the endface of the tip seat by means of a cap screw 40.

[0030] The exchangeable cutting tip 12 can in the exemplary embodimentaccording to FIGS. 1 to 7 be chucked in the tip seat 14 by means of adowel pin 44 extending through an axial bore 42 of the base body 10. Thedowel pin 44 rests for this purpose with its head 46 against the endface surface of the exchangeable cutting tip 12. The dowel pin 44 has atits end not facing the head 46 an external thread 48, onto which thechucking nut 50 illustrated in FIGS. 7a to c can be screwed. Thechucking occurs by tightening of the head 52 of the chucking nut 50against the rearward end face of the base body 10 (FIG. 1a). The dowelpin 44 and the chucking nut 50 each have an axial bore 54, the bore 54in the dowel pin terminating radial channels in the area of the dowelpin head 46. Cooling lubricant is supplied to the cutting edges 16through the axial bore 54 and the radial channels 56.

[0031] The auto-centering fitting of the exchangeable cutting tips 12 inthe tip seat 14 is made possible by the two components of the assemblyabutting without initial tension prior to an engagement of the twoplanar surfaces 22 and 30 so that an elastic expansion of the clampingmembers 32 occurs during a chucking operation. This is made possible onthe one hand by the two planar surfaces 22 and 30 being spaced from oneanother in the assembled condition, however, non-tensioned state withthe key and inclined surfaces 26, 28 being parallel (FIG. 9a). The samepurpose can be achieved by the slope angle of the inclined surfaces 28on the side of the base body being steeper compared to the axis of thebase body than the slope angle of the key surfaces 26 (FIG. 10a). Bysuitably dimensioning the connecting components, a surface bearingpressure indicated by the arrows 58 and a force occurs in both cases dueto the elastic bending of the clamping members 32, and at the same timean abutment in the area of the planar surfaces 22, 30. In order toguarantee a specific expansion, radially and/or axially open expansionslots 36 are formed into the base area of the clamping members 32. Ahigh degree of coaxial alignment of the exchangeable cutting tip 12 andthe base body 10 is achieved by these precautionary measures.

[0032] The base body diagrammatically illustrated in FIGS. 11a to cdiffers from the exemplary embodiment according to FIG. 2 in the designof the end region of the tip seat 14.

[0033] The tip seat 14 according to FIG. 11a has a conical socket 60 inthe planar end face surface 30. The tip seat is thereby formed by sixclamping members 32, which are arranged at equal angular distances fromone another and are separated from one another by axially open spacinggaps 34, the end face surfaces of which clamping members 32 definetogether a plane forming the planar end face surface 30. When a reaminghead with a coaxially projecting outer conical surface is inserted intothis tip seat, the clamping members 32 are elastically expanded relativeto the base body 10 under the action of the axially applied force. Anauto-centering of the exchangeable cutting tip 12 on the base body 10results therefrom.

[0034] The exemplary embodiment illustrated in FIG. 11b shows instead ofthe conical socket 60 a hexagon socket 62. When the shoulder 24 of theexchangeable cutting tip 12 has in this case an outer hexagon surfacecomplementary to the hexagon socket 62, play-free driving rotation willoccur. The spacing gap 34, with which the clamping members 32 areseparated from one another, are arranged in this case at the mutuallyadjacent corners of the hexagon socket 62.

[0035] The exemplary embodiment illustrated in FIG. 11c has a conicalsocket 60 and a hexagon socket 62 in addition to a fitting cylinder 64.The conical socket 60 has here the function of a feed cone for theshoulder of the exchangeable cutting tip, which shoulder has acorresponding fitting cylinder. Furthermore, the spacing gaps 34 aremissing here so that only a relatively small radial expansion over theconical socket 60 and the fitting cylinder 64 can occur during thechucking of the exchangeable cutting tip.

[0036] A high exchange precision is achieved by the grinding work of allexchangeable cutting tips occurring on one and the same master basebody.

[0037] In summary, the following is to be stated: the invention relatesto a machine reamer with a shaft-like base body 10 and a reaming head 12projecting axially from the end face beyond the base body. In order toachieve, in spite of low material and construction costs, a high degreeof precision in coaxialism, it is suggested according to the inventionthat the reaming head is designed as a one-piece exchangeable cuttingtip 12, which can be chucked into an end face tip seat 14 of the basebody 10 and in a coaxially auto-centering manner.

What is claimed is:
 1. A machine reamer comprising a shaft-like basebody (10) and a reaming head projecting axially from the end face of thebase body, characterized in that the reaming head is designed as aone-piece exchangeable cutting tip (12), which can be chucked in anauto-centering manner into an end face of a radially expandable tip seat(14) of the base body.
 2. The machine reamer according to claim 1,characterized in that the exchangeable cutting tip (12) has a shoulder(24), which projects axially beyond a rearward facing planar surface(22), and which has at least three key surfaces (26), which are arrangedat preferably equal angular distances from one another, and supplementone another to form a regular frustum, and that the tip seat (14) has atleast three inclined surfaces (28), which are arranged at preferablyequal angular distances from one another, and are complementary to thekey surfaces (26) of the flat sides of the regular frustum, andterminate in a planar end surface (30), against which axially abuts therearward facing planar surface (22) of the exchangeable cutting tip (12)in the assembled state.
 3. The machine reamer according to claim 2,characterized in that the tip seat (14) has at least three clampingmembers (32), which are arranged at preferably equal angular distancesfrom one another, are separated by axially and/or radially open spacinggaps (34), and each have one of the inclined surfaces (28), the surfacesof which clamping members together clamp in a plane defined by theplanar end surface (30).
 4. The machine reamer according to claim 3,characterized in that the clamping members (32) can be elasticallyexpanded relative to the axis of the base body under the action of anaxially applied force.
 5. The machine reamer according to one of theclaims 1 to 4, characterized in that the base body (10) has inside ofthe tip seat (14) at least three expansion slots (36), which arearranged at preferably equal angular distances from one another, and areopen axially and/or radially toward the end face.
 6. The machine reameraccording to claim 5, characterized in that the expansion slots (36) arearranged in the base area of the respective clamping members (32). 7.The machine reamer according to one of the claims 2 to 6, characterizedin that the slope angle of the key surfaces (26) is 5° to 10° relativeto the axis of the base body.
 8. The machine reamer according to one ofthe claims 2 to 7, characterized in that the slope angle of the inclinedsurfaces (28) on the side of the base body is smaller than the slope ofthe key surfaces (26) on the side of the tips.
 9. The machine reameraccording to one of the claims 2 to 7, characterized in that the slopeangles of the inclined surfaces (28) on the side of the base body and ofthe key surfaces (26) on the side of the tips are the same, and that theplanar surfaces (22, 30) on the end of the base body and on the end ofthe tips are spaced at a defined distance from one another in an initialjoining position of the connecting partners.
 10. The machine reameraccording to claim 1, characterized in that the auto-centering tip seat(14) has a conical socket (60) in the planar end surface (30) of thebase body, and has an inner polygon-sided socket (62) arranged betweenthe conical socket and the planar end surface (30), that theexchangeable cutting tip (12) has an outer conical section, whichprojects axially beyond an annular rearward facing planar surface (22),and can be inserted into the conical socket (60), and has an outerpolygon-sided socket, which is oriented between the outer conicalsection and rearward facing planar surface, and is complementary to theinner polygon-sided socket (62), and that the exchangeable cutting tip(12) abuts with its rearward axially facing planar surface (22) in theassembled state against the planar end surface (30) of the base body(10).
 11. The machine reamer according to claim 10, characterized inthat the tip seat (14) has at least two clamping members (32), which arearranged at preferably equal angular distances from one another, andwhich are separated from one another by axially and/or radially openspacing gaps (34) arranged in the corners of the inner polygon-sidedsocket (62), the end surfaces of which clamping members (32) definetogether a plane forming the planar end surface (30), and which can beelastically expanded relative to the axis of the base body under theaction of the applied force.
 12. The machine reamer according to claim1, characterized in that the auto-centering tip seat (14) has a conicalsocket (60), which is arranged in the planar end surface (30) of thebase body (10), that the exchangeable cutting tip (12) has an outerconical section which projects axially beyond a rearward facing planarsurface (22), and can be placed into the conical socket (60), that thetip seat (14) has at least two clamping members (32), which are arrangedat preferably equal angular distances from one another, and areseparated from one another by axially and/or radially open spacing gaps(34), and the end surfaces of which clamping members (32) togetherdefine a plane forming the planar end surface (30), that theexchangeable cutting tip (12) rests with its rearward facing planarsurface (22) in the assembled state against the planar end surface (30)of the base body (10), and that the clamping members (32) can beelastically expanded relative to the axis of the base body under theaction of the applied force.
 13. The machine reamer according to claim1, characterized in that the auto-centering tip seat (14) has a fittingcylinder (64), in the planar end surface (30) of the base body (10), andhas an inner polygon-sided socket (32), which is arranged between thefitting cylinder (64) and the planar end surface (30), that theexchangeable cutting tip (12) has a cylindrical fitting pin, whichprojects beyond a rearward facing planar surface, can be placed into thefitting cylinder (64), and is over dimensioned relative to the fittingcylinder, and has an outer polygon-sided section which is orientedbetween the fitting pin and the annular planar surface, and iscomplementary to the inner polygon-sided socket (62), and that theexchangeable cutting tip (12) abuts with its rearward axially facingplanar surface in the assembled state against the planar end surface(30) of the base body (10).
 14. The machine reamer according to claim13, characterized in that a conical feed socket (60), which terminatesin the fitting cylinder, is arranged in the tip seat (14) between thefitting cylinder (64) and the inner polygon-sided socket (62).
 15. Themachine reamer according to claim 13 or 14, characterized in that theexchangeable cutting tip (12) has a conical feed section which isarranged near the end face of the fitting pin.
 16. The machine reameraccording to one of the claims 1 to 15, characterized in that theexchangeable cutting tip (12) has several cutting teeth (16), which arearranged spaced from one another.
 17. The machine reamer according toclaim 16, characterized in that the multi-toothed exchangeable cuttingtip has an uneven tooth pitch.
 18. The machine reamer according to oneof the claims 1 to 17, characterized in that the exchangeable cuttingtip (12) can be chucked into the tip seat (14) by means of a centrallyoriented cap screw.
 19. The machine reamer according to one of theclaims 1 to 17, characterized in that the exchangeable cutting tip (12)can be chucked into the tip seat (14) by means of a dowel pin (44),which extends axially through the central part of the base body (10).20. The machine reamer according to claim 19, characterized in that thedowel pin (44) has an axial bore (54), which can be supplied with acooling lubricant.
 21. The machine reamer according to one of the claims1 to 20, characterized in that the exchangeable cutting tip (12)consists of a hard, preferably sintered cutting material, and, ifnecessary, is coated for reducing friction.
 22. The machine reameraccording to claim 21, characterized in that the exchangeable cuttingtip (12) consists of a cutting material of the Group HSS, hard metal,CBN, PKD and ceramics.
 23. The machine reamer according to one of theclaims 10 to 22, characterized in that the inner polygon-sided socket isdesigned as a hexagon socket and the outer polygon-sided section is anouter hexagonal section.
 24. A reaming head for a machine reamer,characterized in that it is designed as a one-piece exchangeable cuttingtip (12), which has a shoulder (24), which axially projects beyond arearward facing planar surface (22), and which has at least three keysurfaces (26), which are arranged spaced at equal angular distances fromone another, and the planes of which supplement one another to form aregular, preferably equal-sided pyramid.
 25. The reaming head for amachine reamer, characterized in that it is designed as a one-pieceexchangeable cutting tip (12), which has a shoulder (24), which axiallyprojects beyond a rearward facing planar surface (22), and which has anouter conical section and an outer polygon-sided section arrangedbetween the outer conical section and the planar surface (22).
 26. Thereaming head for a machine reamer, characterized in that it is designedas a one-piece exchangeable cutting tip (12), which has a shoulder (24),which axially projects beyond a rearward facing planar surface (22), andwhich has a cylindrical fitting portion and an outer polygon-sidedsection arranged between the fitting portion and the planar surface(22).
 27. The reaming head according to claim 26, characterized in thatthe shoulder has a conical feed section passing over into thecylindrical fitting portion.
 28. The reaming head according to one ofthe claims 25 to 27, characterized in that the outer polygon-sidedsection is designed as an outer hexagon section.
 29. The reaming headaccording to one of the claims 24 to 28, characterized in that itconsists of a hard, preferably sintered cutting material.
 30. Thereaming head according to claim 29, characterized in that theexchangeable cutting tip (12) consists of a cutting material of theGroup HSS, hard metal, CBN, PKD and ceramics.
 31. The reaming headaccording to claims 24 to 30, characterized in that the exchangeablecutting tip (12) is multi-toothed.
 32. The reaming head according to oneof the claims 24 to 31, characterized in that the exchangeable cuttingtip has a cylindrical, crowned or conical guide portion (20), whichaxially follows a chamfered portion (18), and which is shorter than halfof the tip diameter.
 33. The reaming head according to one of the claims24 to 32, characterized in that the exchangeable cutting has a centralhole (42) therethrough.